According to a recent article it is not "What difference a day makes" as in the song from Dinah Washington, but it is the difference between voluntary and mandatory.
With ISSB the companies for instance do not have to disclose how they tackle climate and other risks and opportunities related to sustainability. Also a pathway how to reach the goal of net zero or the 1.5 degrees is not a must, but appreciated.
CSRD on the other side will be a must and companies will have to face fines if not complying.